diff --git a/platform/internal/middleware/tenant_guard.go b/platform/internal/middleware/tenant_guard.go
index 3bcd010d..78309bba 100644
--- a/platform/internal/middleware/tenant_guard.go
+++ b/platform/internal/middleware/tenant_guard.go
@@ -81,6 +81,13 @@ func TenantGuardWithOrgID(configuredOrgID string) gin.HandlerFunc {
 			c.Next()
 			return
 		}
+		// Tertiary: same-origin Canvas requests on tenant EC2 instances where
+		// Caddy serves Canvas (:3000) and API (:8080) under the same domain.
+		// CANVAS_PROXY_URL is set → Referer/Origin matches Host → trusted.
+		if isSameOriginCanvas(c) {
+			c.Next()
+			return
+		}
 		// 404 not 403 — existence of this tenant must not be inferable by
 		// probing other orgs' machines.
 		c.AbortWithStatus(404)
diff --git a/platform/internal/middleware/tenant_guard_test.go b/platform/internal/middleware/tenant_guard_test.go
index f82f75ad..01341c25 100644
--- a/platform/internal/middleware/tenant_guard_test.go
+++ b/platform/internal/middleware/tenant_guard_test.go
@@ -133,6 +133,64 @@ func TestOrgIDFromReplaySrc(t *testing.T) {
 	}
 }
 
+// Same-origin Canvas bypass: when CANVAS_PROXY_URL is set and Referer matches
+// Host, the request is from the co-served Canvas and should pass through.
+func TestTenantGuard_SameOriginCanvasBypass(t *testing.T) {
+	origActive := canvasProxyActive
+	canvasProxyActive = true
+	defer func() { canvasProxyActive = origActive }()
+
+	r := newGuardedRouter("org-abc")
+
+	req := httptest.NewRequest("GET", "/workspaces", nil)
+	req.Host = "molecule1.moleculesai.app"
+	req.Header.Set("Referer", "https://molecule1.moleculesai.app/")
+	w := httptest.NewRecorder()
+	r.ServeHTTP(w, req)
+
+	if w.Code != 200 {
+		t.Errorf("same-origin canvas: expected 200, got %d", w.Code)
+	}
+}
+
+// Same-origin Canvas bypass via Origin header (WebSocket upgrade path).
+func TestTenantGuard_SameOriginCanvasViaOrigin(t *testing.T) {
+	origActive := canvasProxyActive
+	canvasProxyActive = true
+	defer func() { canvasProxyActive = origActive }()
+
+	r := newGuardedRouter("org-abc")
+
+	req := httptest.NewRequest("GET", "/workspaces", nil)
+	req.Host = "molecule1.moleculesai.app"
+	req.Header.Set("Origin", "https://molecule1.moleculesai.app")
+	w := httptest.NewRecorder()
+	r.ServeHTTP(w, req)
+
+	if w.Code != 200 {
+		t.Errorf("same-origin canvas via Origin: expected 200, got %d", w.Code)
+	}
+}
+
+// Same-origin Canvas bypass must NOT work when CANVAS_PROXY_URL is unset.
+func TestTenantGuard_SameOriginCanvasInactiveWithoutEnv(t *testing.T) {
+	origActive := canvasProxyActive
+	canvasProxyActive = false
+	defer func() { canvasProxyActive = origActive }()
+
+	r := newGuardedRouter("org-abc")
+
+	req := httptest.NewRequest("GET", "/workspaces", nil)
+	req.Host = "molecule1.moleculesai.app"
+	req.Header.Set("Referer", "https://molecule1.moleculesai.app/")
+	w := httptest.NewRecorder()
+	r.ServeHTTP(w, req)
+
+	if w.Code != 404 {
+		t.Errorf("same-origin canvas without CANVAS_PROXY_URL: expected 404, got %d", w.Code)
+	}
+}
+
 // The allowlist is exact-match, not prefix. "/health/debug" must NOT bypass.
 func TestTenantGuard_AllowlistIsExactMatch(t *testing.T) {
 	gin.SetMode(gin.TestMode)
diff --git a/platform/internal/middleware/wsauth_middleware.go b/platform/internal/middleware/wsauth_middleware.go
index 5b06c576..f1c0711c 100644
--- a/platform/internal/middleware/wsauth_middleware.go
+++ b/platform/internal/middleware/wsauth_middleware.go
@@ -220,19 +220,25 @@ func isSameOriginCanvas(c *gin.Context) bool {
 	if !canvasProxyActive {
 		return false
 	}
-	referer := c.GetHeader("Referer")
-	if referer == "" {
-		return false
-	}
 	host := c.Request.Host
 	if host == "" {
 		return false
 	}
-	// Referer must start with https://<host>/ or http://<host>/ (trailing
-	// slash required to prevent hongming-wang.moleculesai.app.evil.com from
-	// matching hongming-wang.moleculesai.app).
-	return strings.HasPrefix(referer, "https://"+host+"/") ||
-		strings.HasPrefix(referer, "http://"+host+"/") ||
-		referer == "https://"+host ||
-		referer == "http://"+host
+	// Check Referer first (standard browser requests).
+	referer := c.GetHeader("Referer")
+	if referer != "" {
+		// Referer must start with https://<host>/ or http://<host>/ (trailing
+		// slash required to prevent hongming-wang.moleculesai.app.evil.com from
+		// matching hongming-wang.moleculesai.app).
+		if strings.HasPrefix(referer, "https://"+host+"/") ||
+			strings.HasPrefix(referer, "http://"+host+"/") ||
+			referer == "https://"+host ||
+			referer == "http://"+host {
+			return true
+		}
+	}
+	// Fallback: check Origin header (WebSocket upgrade requests may not have
+	// Referer but always send Origin).
+	origin := c.GetHeader("Origin")
+	return origin == "https://"+host || origin == "http://"+host
 }